Injection molding method with compression for hardening

ABSTRACT

A predetermined amount of melted thermoplastic resin of raw material is injected into a molding space which is previously set to a temperature higher than that at which the resin beings to harden under atmospheric pressure and which has a capacity larger than a volume of a product. The injected resin is cooled in the molding space and pressurized before the resin is cooled to the temperature at which the resin begins to harden under atmospheric pressure. When pressurized, the glass transition temperature of the thermoplastic resin is shifted to a higher temperature so that the thermoplastic resin hardens in slight reduction of the temperature. The resin is cooled in the pressurized state until the resin possesses the dynamic rigidity under room temperature and atmospheric pressure. While the thermoplastic resin is further cooled to the extraction temperature, the pressure applied to the resin is controlled until a further increase of the dynamic rigidity due to the cooling ceases and the dynamic rigidity of the thermoplastic resin in the cooling is maintained to that under room temperature and atmospheric pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an injection molding method withcompression and an apparatus therefor, and more particularly to aninjection molding method and apparatus with compression suitable formolding a product such as, for example, an optical lens which requireshigh molding accuracy and uniformity in the compression thereof.

2. Description of the Prior Art

In order to obtain a precise molded product by using thermoplasticresin, the injection molding method with compression has latelyattracted attention.

Thermoplastic resin such as, for example, PMMA (methacrylic resin)increases its dynamic rigidity to harden by cooling the thermoplasticresin from the melted state having a high temperature, and the volume ofthe resin is reduced with a decrease of the temperature.

More particularly, the thermoplastic resin, when cooled, hardens to froma solid product, while only simple cooling and hardening causes failuresin the molded product such as shrinkage and warpage due to a decrease ofthe volume in the solid state thereof as compared with that in themelted state thereof.

Accordingly, a compression margin corresponding to the contraction ratein the molding is generally provided in the joint surface of the mold.Then, after injection of the melted thermoplastic resin into the mold,the thermoplastic resin is cooled to harden in the applied state with amold clamping force.

Various process control methods for the injection molding withcompression have been proposed heretofore. Basically, the temperature ofthe mold is previously set in the vicinity of the extraction temperaturein order to increase the cooling efficiency and the mold is pressurizedimmediately after completion of the injection so that the capacity inthe mold is equal to the volume of the molded product under the normaltemperature and pressure. Then, the pressurized force is controlled tobe gradually reduced with the contraction due to the cooling.

Namely, in the conventional process control, in order to improve theshape and dimensions effected by the contraction due to cooling, thepressure applied to the mold is controlled so that the thermoplasticresin to be molded maintains the volume under room temperature andatmospheric pressure in the whole range of temperature in the cooling.Thus, a satisfactory molded product can be obtained with stability ofthe shape and dimension.

The dynamic rigidity of the thermoplastic resin does not increaseuniformly from the melted state thereof of a high temperature to thesolidified state of room temperature and suddenly increases and hardensfrom a certain temperature (the glass transition point Tg).

Accordingly, if any deviation in the temperature occurs in each portionof the thermoplastic resin upon exceeding the glass transition point inthe cooling, partially solidified portions and partially melted portionsare mixedly produced in the thermoplastic resin in the molding space dueto the deviation in the temperature. If the solidified portions and themelted portions are continuously pressurized uniformly, the solidifiedportions are apt to be subjected to plastic deformation and the innercomposition of the molded product is liable to lack uniformity.

It is a matter of course that if the temperature of the mold ispreviously set to a high temperature and the mold is cooled for asufficient time, since the temperature deviation in each portion of theresin is reduced, the above problems are alleviated to a certain extent.In the case of the molded product having a large size in the dimensionor a partially large difference in thickness, if the resin in the moldis cooled slowly so that the temperature deviation does not occur, theoperation efficiency is greatly deteriorated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel injectionmolding method and apparatus with compression which can produce a moldedproduct with excellent uniformity in the inner composition without alarge temperature deviation in each portion of resin even if a timerequired to harden the resin, particularly a time required to exceed theglass transition point is shortened.

As described above, thermoplastic resin such as PMMA possesses aproperty that the dynamic rigidity thereof is increased as the resin iscooled so that the resin hardens and particularly the dynamic rigidityis suddenly increased upon exceeding the glass transition point.Furthermore, the thermoplastic resin of this kind possesses a propertythat the dynamic rigidity thereof is increased and the glass transitionpoint is shifted to a higher temperature by increasing a pressurizedforce thereto even if the temperature of the resin is the same.

This means that the thermoplastic resin such as PMMA hardens at arelatively high temperature (for example, 125° C. in which the resin isin the melted state under atmospheric pressure) when the thermoplasticresin is pressurized with high pressure.

The present invention has been made by utilization of the property thatthe resin hardens at a high temperature when pressurized.

More particularly, in the present invention, the melted thermoplasticresin injected in the molding space is pressurized to be hardened sothat the resin is substantially completely hardened in the range of aslight variation of temperature and consequently the temperaturedeviation in each portion of the resin in hardening is reduced.

Generally, the present invention is premised on the injection moldingmethod with compression in which a predetermined amount of meltedthermoplastic resin is injected into a metal mold formed with a moldingspace and the resin is cooled to obtain a molded product whilecontrolling a pressure applied to the injected thermoplastic resin inthe metal mold.

The metal mold means comprises a stationary metal mold and a movablemetal mold opposed to each other. The movable metal mold can move amonga first position in which the movable mold cooperates with thestationary mold to form a molding space substantially identical with thevolume of the product, a second position in which the movable moldcooperates with the stationary mold to form a molding space larger thanthe volume of the product and a third position in which the movable moldis separated from the stationary mold.

Prior to the injection operation, the movable mold moves to the secondposition to form the molding space larger than the volume of theproduct. Furthermore, the temperature of the metal mold is initially setto a temperature higher than a temperature at which temperature thethermoplastic resin begins to harden under atmospheric pressure by atemperature control means.

The melted thermoplastic resin is measured by measuring and injectingmeans and a predetermined amount thereof is injected into the moldingspace through a gate means. The injected resin is rapidly cooled to theinitially set temperature of the mold by heat exchange with the mold.

A pressure control means increases the pressure applied to thethermoplastic resin in the metal mold before the injected thermoplasticresin is cooled to the temperature at which temperature the resin beginsto harden under atmospheric pressure. Since the temperature of the metalmold is previously set to a temperature which is higher than thetemperature at which temperature the resin begins to harden underatmospheric pressure, the thermoplastic resin in the mold is maintainedin the melted state in the beginning of the application of the pressureand accordingly the pressurizing force is uniformly applied to the wholeof the thermoplastic resin in the mold.

As described above, the thermoplastic resin possesses the property thatthe dynamic rigidity thereof is increased so that the resin is hardenedwhen the pressurizing force is increased even without a reduction of thetemperature of the resin.

Accordingly, if the thermoplastic resin is applied with the pressurizingforce capable of obtaining the dynamic rigidity larger than that of theglass transition point under the temperature condition in the beginningof the application of the pressure, the thermoplastic resin is hardenedfrom the beginning of the application of the pressure without areduction of the temperature (or with slight reduction of thetemperature) and the dynamic rigidity thereof becomes larger than thatof the glass transition point so that the resin is hardened.

Furthermore, in this manner, when the thermoplastic resin in the meltedstate is hardened by the application of the pressure, the deviation ofthe pressurizing force and the deviation of the temperature in eachportion of the resin in the hardening process are extremely small.

The thermoplastic resin hardened by application of the pressure iscooled to the temperature at which temperature the dynamic rigidityunder room temperature and atmospheric pressure is obtained whilemaintaining the pressurizing force.

Since the thermoplastic resin hardened in the pressurized state has atemperature at which temperature the resin is still melted underatmospheric pressure, the dynamic rigidity thereof is reduced and theresin is softened if the pressurizing force is reduced. However, asdescribed above, the thermoplastic resin of this kind possesses aproperty that the dynamic rigidity thereof is increased as the resin iscooled.

Accordingly, the temperature control means reduces the temperature ofthe mold gradually to cool the thermoplastic resin hardened by theapplication of the pressure to the extraction temperature and thepressure control means reduces the pressurizing force so that increaseof the dynamic rigidity due to cooling is canceled. Thus, thethermoplastic resin is molded while maintaining the dynamic rigidityunder room temperature and atmospheric pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a characteristic diagram showing the relationship of thetemperature and dynamic rigidity of PMMA of an example of athermoplastic resin using pressure as a parameter;

FIG. 2 is a sectional view of an injection molding apparatus withcompression of a direct pressure type according to an embodiment of thepresent invention;

FIG. 3 is a sectional view of a compression margin adjustment cylinderof an example of molding space expansion means;

FIG. 4 is a sectional view showing an example of a valve mechanism fordriving the compression margin adjustment cylinder shown in FIG. 3;

FIG. 5 is a sectional view showing an example of a plasticization deviceand a measuring and injecting device;

FIG. 6 is a circuit diagram showing an example of a control system ofthe present invention;

FIG. 7 is a sectional view of gate means;

FIG. 8 is a graph showing a control characteristic in molding; and

FIG. 9 is a flowchart showing the operation of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention is now described in detail withreference to drawings.

FIG. 1 shows the relationship of the temperature and dynamic rigidity ofPMMA of a an example of thermoplastic resin using pressure as aparameter.

FIG. 1, curves a, b, c, d, e and f show the relationship of temperatureand dynamic rigidity at 1, 200, 400, 600, 800 and 1000 bars,respectively. The curve showing the relationship of the temperature andthe dynamic rigidity of PMMA is shifted to a higher temperature by0.025° C. each time the pressure increases by 1 bar.

Furthermore, in FIG. 1, points a₁, b₁, c₁, d₁, e₁ and f₁, are points inwhich PMMA begins to harden at each of the corresponding pressures;points a₂, b₂, c₂, d₂, e₂ and f₂ are points in which PMMA hardens to astate before the glass transition point at each of correspondingpressures; points a₃, b₃, c₃, d₃, e₃ and f₃ are points in which PMMAhardens to a state exceeding the glass transition point at each ofcorresponding pressures, and points a₄, b₄, c₄, d₄, e₄ and f₄ are pointsin which PMMA hardens completely at each of corresponding pressures.

Under the condition of a pressure of 1 bar, PMMA is in the completelymelted state in the temperature range exceeding 130° C.; it begins toharden at about 125° C. when cooled, the dynamic rigidity just beforethe glass transition point is obtained at 120° C.; the dynamic rigidityexceeding the glass transition point is obtained at 115° C., and PMMAhardens completely at about 100° C.

On the other hand, under the condition of a pressure of 1000 bars; PMMAis in the completely melted state in the temperature range exceeding155° C.; it begins to harden at about 150° C. when cooled; the dynamicrigidity just before the glass transition point is obtained at 145° C.;the dynamic rigidity exceeding the glass transition point is obtained at140° C., and PMMA hardens completely at about 125° C.

Namely, a thermoplastic resin such as PMMA completely hardens even undera relatively high temperature when the resin is in a high pressurestate. Accordingly, the present invention has been made by utilizationof the property of the thermoplastic resin that the resin is hardened byapplication of pressure.

In the present invention, the temperature of the metal mold is initiallyset to a temperature higher than a at which temperature thethermoplastic resin begins to harden under atmospheric pressure and thethermoplastic resin is maintained in the melted state upon thecompletion of injection.

After the completion of the injection, the thermoplastic resin ispressurized to be hardened at a high temperature and the resin hardenedby the application of pressure is cooled. At the same time as thecooling, the pressure applied to the thermoplastic resin is controlledso that the dynamic rigidity of the thermoplastic resin during coolingis maintained conatant. Namely, the temperature and the pressure arecontrolled to cool the thermoplastic resin so that the dynamic rigidityof the resin under room temperature and atmospheric pressure ismaintained.

FIG. 2 is a sectional view of the injection molding apparatus withcompression according to an embodiment of the present invention.

The injection molding apparatus according to the present invention isformed with a frame 1 in the form of a box in a whole configuration andthe frame 1 is divided into chambers 2, 3, 4 and 5 by partition walls.

The chamber 2 constitutes an oil tank which is filled with oil and inwhich an oil hydraulic pump 6 is contained.

The chamber 3 contains a motor 7 for driving the oil hydraulic pump 6,which is coupled with the motor 7 through a through hole formed in apartition wall 8 between the chambers 2 and 3. The oil hydraulic pump 6and the motor 7 constitute a known oil hydraulic unit to feed oil to alloil hydraulic devices.

A mold clamping device 9 is mounted above the chamber 4.

A stationary die-plate 10 is fixedly mounted on the upper wall of thechamber 4. A cylinder fixing plate 13 on which a mold clamping cylinder12 is mounted is fixedly mounted at upper ends of tie-bars 11 fixedvertically in four corners of the stationary die-plate 10. A movabledie-plate 15 is mounted to a piston rod 14 of the mold clamping cylinder12 to be able to move up and down along the tie-bars 11.

Furthermore, a stationary metal mold 16 is exchangeably mounted on thestationary die-plate 10 and a movable metal mold 17 is also exchangeablymounted to the movable die-plate 15.

Compression margin forming cylinders 18 which form a predeterminedcompression margin in the joint surface of the mold between thestationary and movable molds before the injection operation are providedin each of the tie-bars 11. The compression margin means a gap formedpreviously in the joint surface of the mold in consideration ofreduction of the volume of the resin which occurs when the melted resinis compression molded.

FIG. 3 is an enlarged sectional view showing the compression marginadjusting cylinder 18, in which the same elements as those of FIG. 2 aregiven the same numerals.

A bolt or thread 11a is formed at the lower end of the tie-bar 11 andpenetrates a through hole 10a formed in the stationary die-plate 10. Anut 18a which is fitted onto the bolt 11a is formed in the upper surfaceof the compression margin adjusting cylinder 18. The bolt 11a is turnedtightly into the nut 18a so that the tie-bars 11 and the adjustingcylinder 18 are fixed to the stationary die-plate 10.

A chamber 18b is formed in the lower portion of the adjusting cylinder18 and a piston 18c is disposed in the chamber 18b so that the piston18c can move vertically. A lower opening of the chamber 18b is closed bya cap 18e having a through hole which a piston rod 18d penetratesslidably. Numeral 18f denotes a port of the adjusting cylinder 18.

A bottomed cylindrical case 18g includes a nut 18h formed in the centralbottom of the case 18g. The nut 18h is fitted onto a bolt 18i formed inthe lower end of the piston rod 18d.

A pin 19 is inserted into a guide hole 10b formed in the stationarydie-plate 10 vertically movably and a lower end of the pin 19 issupported on an upper end surface of the case 18g.

A spacer ring 20 is mounted around the tie-bar 11 so that the ring 20can move up and down along the tie-bar 11. Accordingly, when oil is fedthrough the port 18f to move back the piston 18c into the chamber 18b,the piston rod 18d, the case 18g, the pin 19 and the spacer ring 20 areintegrally moved up.

On the other hand, cylindrical intermediate members 21 are fixedlymounted in the lower surface of the movable die-plate 15 to cover theouter periphery of the tie-bars 11. Thus, when the spacer ring 20 ispushed up as described above, the movable die-plate 15 is also pushed uptogether with the intermediate members 21 to form a compression margin.The intermediate members 21 have a length which is sufficient so as notto prevent the mold clamping operation when the spacer ring 20 islowered and to form the compression margin when the spacer ring 20 ismoved up.

The compression margin adjusting cylinder 18 and its associatedmechanism serve to set a precise compression margin in order to obtain aprecise molded product.

More particularly, the piston rod 14 is advanced to minimize thecompression margin to zero. At this condition, when the compressionmargin adjusting cylinder 18 is moved against the mold clamping cylinder12, a predetermined compression margin is formed. After the compressionmargin reaches a set value, thermoplastic resin is injected into themold. After completion of the injection, the pressure applied to theadjusting cylinder 18 is reduced and the piston rod 14 is advanced sothat the mold clamping operation is performed.

The present embodiment is characterized in that an oil hydraulic circuitcommunicating with the port 18f of the adjusting cylinder 18 iscompletely closed at the timing when the compression margin reaches theset value so that the adjusting cylinder 18 functions substantially inthe same manner as a so-called mechanical lock to thereby fix thecompression margin precisely.

Accordingly, a valve mechanism for operating the compression marginadjusting cylinder 18 is required to be able to control a small amountof flow and have a high speed responce upon closure.

FIG. 4 is a sectional view showing an example of the valve mechanism foroperating the compression margin adjusting cylinder 18. The valvemechanism 22 discharges oil stepwise in response to pulses produced froma pulse oscillator 23.

The valve mechanism 22 shown in FIG. 4 includes flow ways 22c and 22dformed in parallel with each other between an inlet 22a and an outlet22b and is adapted to pass oil by opening the flow ways 22c and 22dalternately for a short time in synchronism with the pulses producedfrom the pulse oscillator 23.

More particularly, a drive pin 22e made of soft magnetic iron isswingably supported by an axis 22f in a magnetic field. Each time thepulses of the oscillator 23 are supplied to coils 22g and 22h woundaround the drive pin 22e, the polarity of the drive pin 22e is reversedso that the pin is swung.

In the state shown in FIG. 4, a ball 22k in a pilot valve 22j is pushedup together with a pin 22l by pilot pressure, supplied from a pilotpressure source 22i and the pilot pressure is supplied to pressurechambers 22m and 22n.

Accordingly, since a poppet 22o opens a valve seat 22p while a poppet22q closes a valve seat 22r, the flow way 22c is closed as a whole.Furthermore, since a poppet 22s opens a valve seat 22t while a poppet22u closes a valve seat 22v, the flow way 22d is closed as a whole. Oildoes not flow from the inlet 22a to the outlet 22b.

In this state, when the polarity of the pulses produced from the pulseoscillator 23 is reversed, the drive pin 22e is rotated clockwise in thefigure since the right and left polarities of the drive pin 22e arereversed. Accordingly, a ball 22x in a pilot valve 22w is pushed uptogether with a pin 22y by pilot pressure supplied from the pilotpressure source 22i and the pilot pressure is supplied to pressurechambers 22A and 22B.

Thus, the pilot pressure supplied to the pressure chamber 22A causes thepoppet 22o to close the valve seat 22p and the poppet 22u to open thevalve seat 22v while the pilot pressure supplied to the pressure chamber22B causes the poppet 22s to close the valve seat 22t and the poppet 22qto open the valve seat 22r. However, since an orifice 22D is providedbetween the pilot valve 22w and the pressure chamber 22B, a time delayoccurs from the opening of the valve seat 22v by the poppet 22u to theclosure of the valve seat 22t by the poppet 22s, oil flows through theflow way 22d from the inlet 22a to the outlet 22b.

When the polarity of the pulses produced from the pulse oscillator 23 isreversed again, the pilot pressure is supplied to the pressure chambers22m and 22n again and the valve mechanism 22 is returned to the stateshown in FIG. 4. However, since an orifice 22E is provided between thepilot valve 22j and the pressure chamber 22n, a time delay occurs fromthe opening of the valve seat 22p by the poppet 22o to the closure ofthe valve seat 22r by the poppet 22q. The flow way 22c is opened duringthe time delay and accordingly oil flows through the flow way 22c fromthe inlet 22a to outlet 22b.

In this manner, since the valve mechanism shown in FIG. 4 causes oil toflow from the inlet 22a to the outlet 22b only during the time delay setby the orifices 22D and 22E each time the polarity of the pulsesproduced from the oscillator 23 is reversed, the rate of flow iscontrolled exactly as a whole in accordance with a frequency of thepulse and the oil hydraulic circuit extending from the inlet 22a to theoutlet 22b is completely closed by means of the poppets by ceasing thepulse oscillator 23 to thereby satisfy the requirement of the presentinvention.

In FIG. 2, numeral 30 denotes a measuring and injecting device whichincludes a nozzle directed upward. Numeral 40 denotes a plasticizingdevice which plasticizes resin of raw material to feed in to themeasuring and injecting device 30. The measuring and injecting device 30is disposed in the chamber 4 and the plasticizing device 40 is disposedin the chamber 5. The device 30 is coupled with the plasticizing device40.

In the embodiment, the plasticizing device 40 is put on a truck 40a. Theplasticizing device 40 moves together with the measuring and injectingdevice 30 along a rail 40b and rotates about an axis 40c on a verticalplane. A nozzle 31 is positioned in accordance with the movement of thetruck 40a and is coupled with a sprue bush 10c of the stationarydie-plate 10 by the rotation about the axis 40c.

The measuring and injecting device 30, the plasticizing device 40 andthe associated mechanism thereof are required to be able to measure theresin melted in the proper temperature exactly and inject the meltedresin. FIG. 5 shown in section an actual example of the measuring andinjecting device 30 and the plasticizing device 40.

In FIG. 5, the same elements as those described above are given the samenumeral as that of the elements described above.

A plunger 33 is inserted into a lower opening of an injection cylinder32 and is moved up and down by oil hydraulic cylinder 34.

A check valve 35 is provided in a tip of the injection cylinder 32.

An inner diameter of a lower inner periphery 32a of the injectioncylinder 32 positioned near the oil hydraulic cylinder 34 is smallerthan that of an upper inner periphery 32b thereof positioned near thenozzle 31 and a step is formed between the lower inner periphery 32a andthe upper inner periphery 32b of the injection cylinder 32. The lowerinner periphery 32a of the injection cylinder 32 and an outer periphery33a of the plunger 33 are sealed each other. A gap is formed between theupper inner periphery 32b and the outer periphery of the plunger 33 andthe plasticized resin flows through the gap into the injection cylinder32.

The plunger 33 is moved by a difference between a pressure of the resinin the injection cylinder 32 and a pressure of the oil hydrauliccylinder 34.

Numeral 36 represents a heater for heating the injection cylinder 32,TS₁ represents a temperature sensor which detects a temperature of theresin in the injection cylinder 32, and PS₁ represents a pressure sensorwhich detects a pressure in the injection cylinder 32. Numeral 37represents a position sensor of, for example, an optical type whichdetects a backward distance of the plunger 33. An amount of resininjected at one injection operation is measured on the basis of outputsof the temperature sensor TS₁, the pressure sensor PS₁ and the positionsensor 37. As long as the position sensor 37 can detect the backwarddistance of the plunger 33, the disposition thereof is not limited tothe position shown in FIG. 5.

The plasticizing device 40 serves to melt the resin of raw material fedinto a plasticizing cylinder 42 through a hopper 41 by heating it by theheater 43. A screw 44 is rotated by an oil hydraulic motor 45.

A poppet 46 is disposed in the flow path of the resin extending from theplasticizing cylinder 42 to the injection cylinder 32 and is driven by acylinder 47 to open and close the flow path between the plasticizingcylinder 42 and the injection cylinder 32.

The mechanism described above is controlled by a system as shown in FIG.6, for example.

In FIG. 6, the elements described above are given the same numeral asthose described above and description thereof is omitted. Only elementswhich are not described above are described.

Numeral 50 denotes a valve mechanism including a known servo-valve of anelectric-to-hydraulic conversion type and a known pressure controlvalve. The valve mechanism 50 is coupled with ports 12a and 12b of themold clamping cylinder 12. A piston rod 14 is advanced by feeding oil tothe port 12a through the valve mechanism 50.

Similarly, numeral 51 denotes a valve mechanism including a knownservo-valve of an electric-to-hydraulic conversion type and which iscoupled with ports 34a and 34b of the injection cylinder 14. Theinjection operation is performed by feeding oil to the port 34a throughthe valve mechanism 51.

Numeral 52 denotes a directional control valve of the solenoid controltype which is employed to feed oil to the cylinder 47, which is coupledwith any of a pressure source or a drain in accordance with a conditionof the directional control valve 52.

Numeral 53 denotes a shutoff valve for shutting off the oil hydrauliccircuit of the compression margin forming cyrender 18.

Numeral 54 denotes a photointerrupter constituting an example of asensor for detecting the compression margin. Pulses produced from thephotointerrupter 54 in accordance with backward movement of the movabledie-plate 15 are supplied to a controller 56.

Numeral 55 denotes a heater which is used to determine a temperature ofthe resin in the metal mold. The composition and the shape of the heater55 is different depending on the shape of the metal mold. Numeral 57denotes a known input device, 58 a memory and 59 an auxiliary memory.

TS₂ is a temperature sensor for detecting a temperature of the resin inthe mold, PS₂ is a pressure sensor for detecting a pressure of the resinin the mold, and TS₃ is a temperature sensor for detecting a temperatureof the resin in the plasticizing cylinder 42.

Further, numeral 60 denotes a solenoid valve which seals a gate providedin the mold 16. A more actual configuration is shown in an enlarged viewof FIG. 7.

The solenoid valve 60 is coupled with a cylinder 61. The cylinder 61advances a rod 62 in responce to excitation of the solenoid valve 60 toseal the gate and the cylinder 61 move the rod 62 back in response todeenergization of the solenoid valve 60 to open the gate.

The operation of the embodiment is described below with reference to theforegoing.

The operation of the injection molding apparatus with compressionaccording to the present embodiment is divided into (1) the plasticizingoperation of resin by the plasticizing cylinder 42, (2) the compressionmargin forming operation by the mold clamping cylinder 12 and thecompression margin forming cylinder 18, (3) the injection operation bythe injection cylinder 32 and (4) the compression molding operation bythe mold clamping cylinder 12 and the metal molds 16, 17. The operationis described in the order described above. The flowchart shown in FIG. 9would facilitate the understanding of the operational sequence.

First of all, various data such as, for example, a temperature of resinbefore injection, an amount of resin injected by one molding operation,an injection pressure, an injection speed, the magnitude of thecompression margin, a series of data concerning temperature in the metalmold, a series of data concerning the mold clamping force and the likeare inputted from the input device 57 and stored in the memory 58 andthe auxiliary memory 59.

The heater 43 is set to 190° C. which is the temperature of PMMA whichis to be injected.

In the initial state, the controller 56 releases excitation of thedirectional control valve 52 so that the cylinder 47 is coupled to thedrain through the directional control valve 52.

Accordingly, the poppet 46 opens to connect the plasticizing cylinder 42to the injection cylinder 32.

The PMMA fed from the hopper 41 is heated by the heater 43 and ismelted.

When the temperature sensor TS₃ detects that a temperature of the PMMAin the cylinder 42 has reached 190° C., the controller 56 drives themotor 45 to rotate the screw 44.

At this time, the valve mechanism 51 keeps the balance of pressurebetween the ports 34a and 34b of the oil hydraulic cylinder 34.Accordingly, the plunger 33 can move up and down freely in the injectioncylinder 32 in accordance with the external pressure.

In the initial state, since the solenoid valve 60 is excited and the rod62 seals the gate, the melted PMMA flows from the plasticizing cylinder42 into the injection cylinder 32 in response to the rotation of thescrew 44 and the plunger 33 is moved back.

As the injection cylinder 32 is filled with PMMA, the plunger 33 ismoved back and the backward amount thereof is detected by the positionsensor 37. When the controller 56 judges that a predetermined amount ofPMMA is stored in the injection cylinder 32 by the fact that the outputof the position sensor 37 reaches a predetermined reference value, thecontroller 56 excites the directional control valve 52.

The cylinder 47 is coupled with the pressure source in response to theexcitation of the directional control valve 52 and the shutoff valvecloses between the plasticizing cylinder 42 and the injection cylinder32. Thus, the predetermined amount of PMMA is stored in the injectioncylinder 32.

As described above, in the embodiment, it is detected by the output ofthe position sensor 32 that the predetermined amount of PMMA is filledin the injection cylinder 32, while the volume of PMMA varies slightlydepending on temperature and pressure.

Accordingly, in the embodiment, the condition for operating the cylinder47 is corrected by the output of the pressure sensor PS₁ and thetemperature sensor TS₁.

More particularly, the volume of PMMA is reduced as the pressure appliedthereto is increased. Accordingly, the controller 56 attains correctionso that the reference value with respect to the detected output of theposition sensor 37 is reduced as the detected pressure of the pressuresensor PS₁ is increased while the reference value with respect to thedetected output of the position sensor 37 is increased as the detectedpressure of the pressure sensor PS₁ is decreased.

Further, the volume of PMMA is reduced as the temperature thereof islower. Accordingly, the controller 56 attains correction so that thereference value with respect to the detected output of the positionsensor 37 is reduced as the detected temperature of the temperaturesensor TS₁ is lowered while the reference value with respect to thedetected output of the position sensor 37 is increased as the detectedtemperature of the temperature sensor TS₁ is raised.

When the predetermined amount of PMMA is filled in the injectioncylinder 32 in the manner described above, the controller 56 controlsthe compression margin forming operation.

In the embodiment, since the compression margin adjusting cylinder 18forms the compression margin against the mold clamping cylinder 12, thevalve mechanisms 50 and 22 are adjusted so that a relationship of P₁ ·A₁<P₂ ·A₂ is satisfied and a difference of the relationship is extremelysmall where the inner diameter of the mold clamping cylinder 12 is A₁,the oil pressure thereof is P₁, the inner diameter of the compressionmargin adjusting cylinder 18 is A₂, and the oil pressure thereof is P₂.

Furthermore, the shutoff valve 53 is deenergized to open the port 18f ofthe compression margin adjusting cylinder 18. The pulse oscillator 23ceases the oscillation operation thereof.

In this state, the controller 56 operates the valve mechanism 50 to feedoil to the port 12a. At this time, since the pressure in the compressionmargin adjusting cylinder 18 is reduced, the piston rod 14 is advancedand the compression margin s between the movable mold 17 and thestationary mold 16 is minimized to zero so that the force of P₁ ·A₁ isapplied to the joint surface between the movable mold 17 and thestationary mold 16.

When the compression margin is minimized to zero, the controller 56excites the shutoff valve 53 and operates the pulse oscillator 23 andreceives pulses produced from the photointerrupter 54.

When the oscillator 23 produces the pulses, the valve mechanism 22discharges oil stepwise in response to each edge of the pulses.Furthermore, the shutoff valve 53 is excited to be closed. Accordingly,the oil discharged by the valve mechanism 22 is fed to the port 18f ofthe compression margin adjusting cylinder 18.

A relationship of P₁ ·A₁ <P₂ ·A₂ is formed between the force P₁ ·A₁produced by the mold clamping cylinder 12 and the force P₂ ·A₂ producedby the compression margin adjusting cylinder 18. Accordingly, the piston18c shown in FIG. 3 is moved back into the chamber 18b. Since the case18g is moved up while pushing up the pin 19, the spacer ring 20, theintermediate member 21 and the movable die-plate 15, the compressionmargin s is increased.

The photointerrupter 54 produces pulses in accordance with the moving upof the movable die-plate 15. The controller 56 adds the pulses producedfrom the photointerrupter 54 to obtain the current value of thecompression margin s. When the current value of the compression margin sreaches the set value of the compression margin s stored in the memory58, the pulse oscillator 23 ceases operation.

As described above, when the valve mechanism 22 is not supplied with thepulses from the oscillator 23, since the flow ways 22c and 22d betweenthe inlet 22a and the outlet 22b are completely cut off by the poppetmechanism and the shutoff valve 53 is also shut off, the path of retreatof the oil fed in the compression margin adjusting cylinder 18 iscompletely intercepted.

Since the mold clamping cylinder 12 adds the force of P₁ ·A₁ to thecompression margin adjusting cylinder 18, the adjusting cylinder 18fixes its length completely while satisfying the relation of P₁ ·A₁ =P₂'·A₂, and the compression margin s is also fixed.

At this time, the pressure of the adjusting cylinder 18 varies from P₂to P₂ ', while the compression of oil in the adjusting cylinder 18 is inthe numerical range in which the compression can be substantiallyneglected as compared with the compression margin s.

In this manner, when a proper compression margin is set, the controller56 controls the injection operation of PMMA.

The temperature of the heater 55 for the metal mold is adjusted to 125°C. which is an example of a temperature before PMMA begins to hardenunder atmospheric pressure.

The temperature of the heater 36 for the injection cylinder 32 isadjusted to 190° C. which is an example of a temperature at whichtemperature PMMA does not begin to harden under any pressure.

Accordingly, PMMA in the injection cylinder 32 is completely melted.

The controller 56 deenergizes the solenoid valve 60 shown in FIG. 7 torelease the sealing of the gate. Thereafter, the controller 56 operatesthe valve mechanism 51 to feed oil to the port 34a of the oil hydrauliccylinder 34. Accordingly, the plunger 33 is advanced in the injectioncylinder 32 so that the melted PMMA is injected into the molding spaceformed by the movable metal mold 17 and the stationary metal mold 16.

The pressure in the adjusting cylinder 18 is slightly varied by theinjection pressure at this time. However, the pressure in the cylinder18 is a reactive force against the force exerted between the stationarydie-plate 10 and the movable die-plate 15 and does not depend on theexternal oil hydraulic circuit. Furthermore, since the injectionpressure is extremely small, the compression margin s is hardly varied.

After completion of the injection, the solenoid valve 60 is excited toseal the gate. Thereafter, the shutoff valve 53 is deenergized to reducethe pressure of the adjusting cylinder 18 so that compression molding ofPMMA can be attained.

The resin injected into the metal mold is rapidly cooled to 125° C. thatis the initially set temperature by the heat exchange with the metalmold.

The detected value of the temperature sensor TS₂ in the metal mold risesat a heat by injecting PMMA heated to 190° C., while the detected valueof the sensor TS₂ is decreased again by cooling the PMMA.

As described above, PMMA does not harden under a pressure of 1 bar untilthe temperature thereof is decreased to about 125° C., while the PMMAbegins to harden at about 150° C. under a pressure of 1000 bars.

In the embodiment, when the temperature sensor TS₂ detects the fact thatthe temperature of the PMMA reaches the temperature at which temperaturethe PMMA does not begin to harden still under a lower pressure but thePMMA begins to harden under application of the pressure, the valvemechanism 50 is controlled to suddenly increase the mold clamping forceof the cylinder 12 so that the PMMA in the metal mold is pressurized ata heat to obtain the dynamic rigidity larger than that of the glasstransition point with a slight reduction of the temperature.

A curve shown by the thick line of FIG. 8 shows an example of a controlcurve of the temperature and the pressure upon hardening.

When PMMA is injected into the metal mold, the temperature of PMMA isreduced to 145° C. after a while from the gate sealing. When the size ofthe product is large, a case where the temperature of the resin ispartially reduced to 145° C. before the gate sealing is considered,while in this case the injection speed is increased or the initialtemperature of the metal mold is set to a temperature higher than 125°C. to control so that each of PMMA is cooled to about 145° C. uniformly.

When PMMA has been cooled about 145° C. under atmospheric pressure, thePMMA has been completely melted and accordingly, a pressure is appliedto each portion of PMMA uniformly.

In the embodiment, while PMMA is in the range of temperature in whichPMMA maintains its completely melted state, the pressure applied to thePMMA is suddenly increased so that the dynamic rigidity thereof issuddenly increased during a slight reduction of the temperature of thePMMA.

When the detected temperature of the sensor TS₂ reaches 145° C. (pointp₁ of FIG. 8), the controller 56 controls the valve mechanism 50 toincrease the pressure applied to the port 12a of the mold clampingcylinder 12 so that pressure of, for example, 600 bars is applied to thePMMA in the metal mold.

Thus, the dynamic rigidity of the PMMA is increased by the appliedpressure thereto as described above and the dynamic rigidity reachesE×10 (dyn/cm²) which is a state just before the glass transition pointwhen PMMA is cooled to 135° C. (point p₂ of FIG. 8). E is a coefficientwhich depends on the resin used.

When the temperature sensor TS₂ detects that the PMMA has been cooled to135° C., the controller 56 controls the valve mechanism 50 to increasethe pressure applied to the port 12a of the mold clamping cylinder 12 sothat a pressure of 1000 bars is applied to the PMMA in metal mold.

This application of pressure further increases the dynamic rigidity ofPMMA and when PMMA has been cooled to 133° C. (point p₃ of FIG. 8) thedynamic rigidity exceeds the glass transition point completely. Whencooled to 130° C. (point p₄ of FIG. 8) the PMMA has been almostcompletely hardened and when cooled to 125° C. (point p₅ of FIG. 8) thePMMA is completely solid.

With the control as described above, since the dynamic rigidity of thePMMA exceeds the glass transition point while the PMMA is cooled in asmall temperature range of 2° C. from 135° C. to 133° C., the wholecooling time is extremely short and the operation efficiency is improvedeven if the cooling speed in the small temperature range is madesufficiently slow so that a deviation in the temperature does not occurin each portion of the PMMA while the glass transition point isexceeded.

The suitable cooling speed in exceeding the glass transition point isdifferent depending on the temperature conductivity of the resin, theshape of a product and the like. When a molded product having a largesize or deviation in thickness is obtained, the temperature of the metalmold is set to a higher temperature or the temperature of the metal moldis reduced stepwise so that the deviation in temperature of each portionof the resin is small.

The PMMA is completely solidified at the temperature of 125° C. in theapplied state of pressure of 1000 bars as described above. However, asapparent from FIGS. 1 and 8, the temperature of 125° C. is a temperatureat which PMMA begins to harden slightly under atmospheric pressure.Accordingly, when PMMA being in the applied state of pressure isreturned to atmospheric pressure state, the PMMA is softened again.Therefore, in order to extract the molded product, PMMA must be cooledto the temperature (that is, 100° C.) at which PMMA is completely solidunder atmospheric pressure.

Accordingly, in the embodiment, the set temperature in the metal mold bythe heater 55 is reduced stepwise to cool PMMA to 100° C. which is theextraction temperature in the mold clamping state in which the pressureis applied to the port 12a of the mold clamping cylinder 12 by the valvemechanism 50, while if the high pressure of 1000 bars is continuouslyapplied to the PMMA in the solid state, plastic deformation occurs inthe molded product.

As described above, when the PMMA has certain dynamic rigidity, thetemperature of the PMMA is varied by 0.025° C. each time the appliedpressure is varied by 1 bar.

Accordingly, the controller 56 controls the valve mechanism 50 whilewatching the detected temperature in the metal mold by the temperaturesensor TS₂. Thus, each time the temperature of the PMMA is reduced by 1°C., the pressure applied to PMMA is reduced by 40 bars so that themolded product is obtained while the dynamic rigidity of the PMMA ismaintained uniform.

The control of temperature and pressure as described above is performedby a known command value following control.

As described above, in the case where the pressure is reduced by 40 barseach time the temperature is reduced by 1° C., the pressure is reducedto 800 bars at 120° C., 600 bars at 115° C., 400 bars at 110° C., 200bars at 105° C. and atmospheric pressure at 100° C.

When the molding operation is completed as described above, thecontroller 56 controls the valve mechanism 50 to feed oil to the port12b of the mold clamping cylinder 12 so that the movable metal mold 17is lifted and the molded product is extracted by a known ejectormechanism not shown.

The essence of the present invention resides in that resin which is inthe melted state under atmospheric pressure but is set to thetemperature at which the resin begins to harden by the application ofpressure is applied with pressure and is hardened to exceed the glasstransition point at a slight reduction of temperature. However, thetemperature of the resin before injection, the initial set temperatureof the metal mold, the temperature in the beginning of the applicationof pressure, the applied pressure and the like are different dependingon the kind of resin, the size of the molded product, the shape of theproduct and the like and the molding time is also different depending onthese condition. It is necessary to decide the optimum moldingconditions for each molded product.

In the above embodiment, reference is made to the actual exampleconcerning the measuring and injecting device and the plasticizingdevice, while the configuration thereof is not limited as long as thetemperature of the resin before injection can be controlled properly andthe amount of resin can be also measured exactly.

Furthermore, the above embodiment makes reference to the mechanism forforming the compression margin, while the configuration thereof is notalso limited thereto as long as the accuracy of adjusting thecompression margin can be satisfied.

As described above, in the present invention, since, when the injectedresin has been cooled to the temperature at which the resin ismaintained in the melted state under atmospheric pressure and begins tobe hardened by the application of pressure, the resin is pressurized tobe hardened to obtain the dynamic rigidity larger than that of the glasstransition point, the resin exceeds the glass transition point while thetemperature thereof is reduced slightly.

Accordingly, since the temperature reduction necessary to exceed theglass transition point in the molding of resin is extremely small, evenif the cooling speed is made slow so that the temperature deviation doesnot occur in each portion of the resin during this process, the timerequired to exceed the glass transition point is small.

According to the present invention, since the initial set temperature ofthe metal mold is set to a temperature higher than that at which resinbegins to be hardened under atmospheric pressure and the resin beforethe beginning of application of pressure is completely melted state, thepressure is applied to each portion of the resin uniformly andsolidified portions and melted portions are not mixedly produced in theresin in the molding. Accordingly, it is hard to produce partiallyplastic deformation and lack of uniformity of the inner composition andthe molded product superior to the uniformity of the inner compositioncan be easily obtained.

Further, in the present invention, since the temperature of the resinand the pressure applied to the resin are controlled so that increase ofthe dynamic rigidity due to the temperature reduction and decrease ofthe dynamic rigidity due to the pressure reduction are canceled eachother while the resin which has been hardened is cooled to thetemperature for extraction, the resin in the cooling maintains aconstant dynamic rigidity and the property of the molded product is notdeteriorated in the cooling.

We claim:
 1. An injection molding method with compression in which apredetermined amount of melted thermoplastic resin is injected into ametal mold formed with a molding space and the thermoplastic resininjected into the metal mold is cooled while controlling a pressureapplied to the resin so that a molded product is obtained,comprising:injecting the predetermined amount of melted thermoplasticresin into the metal mold formed with the molding space larger than avolume of the molded product, a temperature of the metal mold havingbeen previously set to a temperature which is higher than a temperatureat which the resin begins to harden under atmospheric pressure;hardening the thermoplastic resin by increasing the pressure applied tothe thermoplastic resin injected into the metal mold before thethermoplastic resin injected into the metal mold is cooled to thetemperature at which the resin begins to harden under atmosphericpressure; cooling the thermoplastic resin while maintaining the pressureapplied thereto to a temperature at which a dynamic rigidity of thethermoplastic resin reaches a dynamic rigidity of the resin under roomtemperature and atmospheric pressure while maintaining the pressureapplied to the resin; and further cooling the thermoplastic resin to anextraction temperature while also reducing the pressure applied to theresin until a further increase of the dynamic rigidity due to coolingceases, to thereby obtain the molded product.
 2. An injection moldingmethod with compression in which a predetermined amount of meltedthermoplastic resin is injected into a metal mold formed with a moldingspace capable of effecting temperature control and pressure control andthe thermoplastic resin injected into the metal mold is cooled whilecontrolling a pressure applied to the resin so that a molded product isobtained, comprising:injecting the predetermined amount of meltedthermoplastic resin into the metal mold formed with the molding spacelarger than a volume of the molded product, a temperature of the metalmold having been previously set to a temperature which is higher than atemperature at which the resin begins to harden under atmosphericpressure; shifting the glass transition temperature to a highertemperature to harden the thermoplastic resin by increasing the pressureapplied to the thermoplastic resin injected into the metal mold beforethe thermoplastic resin injected into the metal mold is cooled to thetemperature at which the resin begins to harden under atmosphericpressure; cooling the thermoplastic resin while maintaining the pressureapplied thereto to a temperature at which a dynamic rigidity of thethermoplastic resin reaches a dynamic rigidity of the resin under roomtemperature and atmospheric pressure while maintaining the pressureapplied to the resin; and further cooling the thermoplastic resin to anextraction temperature while also reducing the pressure applied to theresin until a further increase of the dynamic rigidity due to coolingceases, to thereby obtain the molded product.
 3. An injection moldingmethod with compression in which a predetermined amount of meltedthermoplastic resin is injected into a metal mold formed with a moldingspace capable of effecting a temperature control and pressure controland the thermoplastic resin injected into the metal mold is cooled whilecontrolling a pressure applied to the resin so that a molded product isobtained, comprising:injecting the melted thermoplastic resin having amass equal to that of the molded product into the metal mold formed withthe molding space larger than a volume of the molded product, atemperature of the metal mold having been previously set to atemperature which is higher than a temperature at which the resin beginsto harden under atmospheric pressure; shifting the glass transitiontemperature to a temperature which is higher than a current temperatureof the thermoplastic resin to harden the thermoplastic resin byincreasing the pressure applied to the thermoplastic resin injected intothe metal mold before the thermoplastic resin injected into the metalmold is cooled to the temperature at which the resin begins to hardenunder atmospheric pressure; cooling the thermoplastic resin whilemaintaining the pressure applied thereto to a temperature at which adynamic rigidity of the thermoplastic resin reaches a dynamic rigidityof the resin under room temperature and atmospheric pressure whilemaintaining the pressure applied to the resin; and further cooling thethermoplastic resin to an extraction temperature while also reducing thepressure applied to the resin until a further increase of the dynamicrigidity due to cooling ceases and the dynamic rigidity of the resin ismaintained at the dynamic rigidity under room temperature andatmospheric pressure, to thereby obtain the molded product.